Set color for UI window border
I agree.  It doesn't exist yet.  

I was pushing for it on the dev site,
and they told me to go to the wiki instead.
https://mobile.twitter.com/doyousketch2/status/835212481614266369
I wasn't about to sign up for another flippin' website
just re-type what I'd already typed in.  

I ended up suggesting it on:
https://code.blender.org/2016/12/the-blender-101-project-and-you
but who knows where that comment went.  

Makes me feel like Blender doesn't want to hear about it.